Constant term of the reduction by x^2->x+1 of the polynomial p(n,x) defined below in Comments.

Terms

a(0) =1a(1) =6a(2) =12a(3) =23a(4) =40a(5) =68a(6) =113a(7) =186a(8) =304a(9) =495a(10) =804a(11) =1304a(12) =2113a(13) =3422a(14) =5540a(15) =8967a(16) =14512a(17) =23484a(18) =38001a(19) =61490a(20) =99496a(21) =160991a(22) =260492a(23) =421488a(24) =681985a(25) =1103478a(26) =1785468a(27) =2888951a(28) =4674424a(29) =7563380